4. Forces and Weight

Question 1

What is mass?

Answer 1

Amount of matter in an object

Question 2

What is weight?

Answer 2

Force of gravity acting on an object

Question 3

What is the equation involving weight, mass and gravitational field strength?

Answer 3

Weight = mass x gravitational field strength

W = mg

Question 4

What is gravitational field strength?

Answer 4

Force between objects that have mass

Question 5

A rocket takes off and accelerates. The rocket will use up its fuel but the thrust of the engine remains constant. How do the following change over time?

1. Thrust
2. Mass
3. Weight
4. Resultant force
5. Acceleration
6. Velocity

Answer 5

1. Thrust = constant
2. Mass = decrease
3. Weight = decrease
4. Resultant force = increase
5. Acceleration = increase
6. Velocity = increase

Question 6

In free fall what is the only force acting?

Answer 6

Weight

In the absence of air resistance all objects will fall with an acceleration equal to g

Question 7

How does air resistance change as you speed up?

Answer 7

Increases

Question 8

What two forces act on skydivers?

Answer 8

• weight which remains constant

- air resistance that increases with speed

Question 9

What are the 3 parts of a free fall question that you have to include?

Answer 9

1. As parachutist jumps:
   - only force acting = weight
   - air resistance = 0
   - acceleration
2. Then:
   - air resistance increases as speed increases
   - resultant downwards force decreases (cos more upwards force)
   - acceleration thus decreases
3. Terminal velocity:
   - air resistance reaches a maximum + is equal to weight
   - resultant force is 0N
   - acceleration = 0
   - travels at constant velocity (terminal velocity)

Question 10

What are the 3 parts of a parachutist question that you have to include?

Answer 10

1. Parachutist at terminal velocity :
   - resultant force + acceleration = 0
   - constant velocity
2. Opens chute:
   - air resistance increases + resultant force is in negative direction
   - thus large deceleration
   - still moves down
3. As it slows, air resistance decreases until air resistance = weight
   - resultant force = 0 and acceleration = 0
   - new slower terminal velocity